Facebook like button, locale not working

The user can change language on my webpage and I wish to change the text of the fb like button according to the current language. I use the iframe method and have read the documentation from fb but this is not working:

<iframe
  src='http://www.facebook.com/plugins/like.php?locale=en-US&amp;href=http%3A%2F%2Fwww.example.com&amp;layout=button_count&amp;show_faces=false&amp;width=80&amp;action=recommend&amp;colorscheme=light&amp;height=10'
  scrolling='no' frameborder='0'
  style='border:none; overflow:hidden;
  width:80px; height:80px;'
  allowTransparency='true'></iframe>

I have set html lang='en-US' and locale=en-US in the src above but still the text display in my windows locale language.

What am I doing wrong?

If i remember right it (the locale= in the iframe address) should be en_US not en-US.

-michael